Image reading apparatus and image reading method

1. An image reading apparatus comprising: N sensor chips arranged in a first direction, N being an integer of two or more, each of the N sensor chips including a plurality of image pickup elements arranged in the first direction; N optical systems that respectively form, on the N sensor chips, reduced-size images of N reading ranges arranged in the first direction on a document so as to form overlap regions that are regions where parts of regions in ends of adjacent reading ranges of the N reading ranges overlap each other; and an image processing section that uses image data in the overlap regions of image data of the N reading ranges arranged in the first direction, thereby obtaining positions of the overlap regions having a highest correlation of the adjacent reading ranges of the N reading ranges, obtains, based on the positions and predetermined synthesis reference positions in the overlap regions, magnifications of read images and synthesis positions that indicate positions where two pieces of the image data are combined, performs image processing of correcting the magnifications in the first direction of the image data of the N reading ranges, and combines the image data of the N reading ranges subjected to the image processing, thereby generating synthesized image data.

2. The image reading apparatus of claim 1 , wherein: the N optical systems that respectively form, on the N sensor chips, reduced-size images of the N reading ranges arranged in the first direction are arranged so as to form the overlap regions that are regions where the parts of regions in the ends of the adjacent reading ranges overlap each other; and each of the N optical systems includes a first lens that optically reduces light reflected on the document, a diaphragm through which part of the light reduced by the first lens passes, and a second lens that forms an image on one of the N sensor chips from the light that has passed through the diaphragm.

3. The image reading apparatus of claim 1 , wherein: the image processing section includes a similarity degree calculator that uses the image data of the overlap regions that are the regions where the adjacent reading ranges of image data of the N reading ranges overlap each other, the image data of the overlap regions being image data generated by the N sensor chips, and thereby calculates a similarity degree indicating a degree of similarity that is a degree of correlation between image data of matching regions set in the overlap regions, a synthesis position estimating unit that estimates the synthesis positions at which two pieces of image data of the adjacent reading ranges are combined, from the similarity degree calculated by the similarity degree calculator, and a synthesizing unit that, based on the synthesis positions estimated by the synthesis position estimating unit, sets the magnifications of the reading ranges and the synthesis positions for the image data of the N reading ranges, converts widths in the first direction of the image data of the N reading ranges, and synthesizes the two pieces of image data having the converted widths, thereby generating the synthesized image data.

4. The image reading apparatus of claim 3 , wherein the similarity degree is a value based on a sum of differential absolute values of pixels in the matching regions set in the overlap regions, and the similarity degree increases as the sum of differential absolute values decreases.

5. The image reading apparatus of claim 3 , wherein the similarity degree is a value based on a sum of squares of differential absolute values of pixels in the matching regions set in the overlap regions, and the similarity degree increases as the sum of squares of differential absolute values decreases.

6. The image reading apparatus of claim 3 , wherein: the synthesizing unit includes a synthesis magnification setting unit that sets the synthesis magnifications for each of the image data of the N reading ranges, based on the synthesis positions estimated by the synthesis position estimating unit and the predetermined synthesis reference position, and outputs the synthesis magnifications and synthesis connection positions, an image converter that converts the widths in the first direction of the image data of the N reading ranges by using the synthesis magnifications set by the synthesis magnification setting unit, and an overlap region connecting unit that synthesizes the image data having the widths converted by the image converter while making the synthesis connection positions set by the synthesis magnifications setting unit coincide with each other, thereby generating the synthesized image data.

7. The image reading apparatus of claim 6 , wherein: the synthesis magnification setting unit includes, with respect to each of the image data of the N reading ranges, a reading width calculator that obtains, from the synthesis positions estimated by the synthesis position estimating unit, a synthesis reading width from a width in the first direction between a synthesis position at an end in the first direction and a synthesis position at another end, and a magnification and synthesis position setting unit that calculates the synthesis magnifications from the width in the first direction between the ends based on the predetermined synthesis reference position in a reading range and the synthesis reading width supplied from the reading width calculator, and obtains the synthesis positions estimated by the synthesis position estimating unit as a synthesis connection position in conversion by using the synthesis magnifications, thereby setting the synthesis magnifications and the synthesis connection position.

8. The image reading apparatus of claim 7 , wherein: the magnification and synthesis position setting unit sets the synthesis magnifications and the synthesis connection position from a ratio between the synthesis reading width supplied from the reading width calculator and the width of the reading range based on the predetermined synthesis reference position, sets the synthesis magnifications at 1 if a synthesis position at one end and a synthesis position at another end in the first direction in each of the image data of the N reading ranges coincide with the predetermined synthesis reference positions in the corresponding one of the image data of the N reading ranges and the synthesis reading width supplied from the reading width calculator is equal to the width of the reading range based on the predetermined synthesis reference position, sets the synthesis magnifications at a value less than 1 if the synthesis position at one end and the synthesis position at another end in the first direction in each of the image data of the N reading ranges are outside the predetermined synthesis reference positions in the corresponding one of the image data of the N reading ranges and the synthesis reading width supplied from the reading width calculator is larger than the width of the reading range based on the predetermined synthesis reference position, and sets the synthesis magnifications at a value greater than 1 if the synthesis position at one end and the synthesis position at another end in the first direction in each of the image data of the N reading ranges are inside the predetermined synthesis reference positions in the corresponding one of the image data of the N reading ranges and the synthesis reading width supplied from the reading width calculator is smaller than the width of the reading range based on the predetermined synthesis reference position.

9. The image reading apparatus of claim 6 , wherein: the synthesis magnification setting unit includes an overlap amount calculator that, for each end in the first direction of the image data of the N reading ranges, calculates a width of an overlap region in the reading range from the synthesis positions estimated by the synthesis position estimating unit, and obtains a synthesis overlap amount, and a magnification and synthesis position setting unit that calculates the synthesis magnifications in the N reading ranges from the synthesis overlap amount at each end of the reading range supplied from the overlap amount calculator and an overlap amount at the predetermined synthesis reference position of reading range, obtains the synthesis positions estimated by the synthesis position estimating unit as a synthesis connection position in conversion by using the synthesis magnifications, and sets the synthesis magnifications and the synthesis connection positions.

10. The image reading apparatus of claim 9 , wherein: the magnification and synthesis position setting unit sets the synthesis magnifications and the synthesis connection position from a ratio between the synthesis overlap amount supplied from the overlap amount calculator and an overlap amount based on the predetermined synthesis reference position, sets the synthesis magnifications at 1 if a synthesis overlap amount at one end in the first direction in each of the image data of the N reading ranges is equal to an overlap amount at the predetermined synthesis reference position, sets the synthesis magnifications at a value less than 1 if the synthesis overlap amount at one end in the first direction in each of the image data of the N reading ranges is smaller than the overlap amount at the predetermined synthesis reference position, and sets the synthesis magnifications at a value greater than 1 if the synthesis overlap amount at one end in the first direction in the image data of the N reading ranges is larger than the overlap amount at the predetermined synthesis reference position.

11. An image reading method that is executed by an image reading device including N sensor chips arranged in a first direction, N being an integer of two or more, each of the N sensor chips including a plurality of image pickup elements arranged in the first direction; and N optical systems that respectively form, on the N sensor chips, reduced-size images of N reading ranges arranged in the first direction on a document so as to form overlap regions that are regions where parts of regions in ends of adjacent reading ranges of the N reading ranges overlap each other; the image reading method comprising the steps of: using image data in the overlap regions of image data of the N reading ranges arranged in the first direction, thereby obtaining positions of the overlap regions having a highest correlation of the adjacent reading ranges of the N reading ranges, obtaining, based on the positions and predetermined synthesis reference positions in the overlap regions, magnifications of read images and synthesis positions that indicate positions where two pieces of the image data are combined, and performing image processing of correcting the magnifications in the first direction of image data of the N reading ranges, and combining the image data of the N reading ranges subjected to the image processing, thereby generating synthesized image data.